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Bristol's most successful aircraft during the inter-war period was the Bristol Bulldog fighter which formed the mainstay of the RAF's fighter-interceptor force from 1930 onwards. In 1932 the RAF were seeking a replacement for entry into service during the 1935-1936 period and the Air Ministry specification F.7/30 demanded a fighter aircraft capable of at least 250 mph and armed with four machine guns. The Bristol Aircraft Company concurrently worked on no less than three designs to meet this challenging specification;

By late 1933 Bristol's Chief Designer, Frank Barnwell, had begun to favour the middle-ground of the Type 125 but acknowledged that the Perseus engine needed more development as the first production versions of the Perseus were rated at a disappointing 580 hp (lower than the same-size Mercury) although future improvements would see the Perseus not only eventually deliver the planned 810 hp by about 1936 but an impressive 930 hp was forecast further down the line.

The Type 125 'Bullfrog' first flew on June 2nd, 1934 piloted by Cyril Uwins and testing over the next eight months proved very successful, especially when an improved and more powerful Perseus engine was installed. In the long-awaited F.7/30 competitive trials held at RAF Martlesham Heath during the Spring of 1935 the Bristol Type 125 and the rival Gloster Gladiator were hard to seperate and the Air Ministry eventually awarded productions contracts to both Bristol and Gloster for 160 aircraft each.

Entering service with the recently formed RAF Fighter Command in March 1937 the Bristol Bullfrog's introduction into RAF service was initially difficult with many pilots being caught out by the aircraft's increased wing loading but it soon became as popular as the earlier Bristol Bulldog had. By the outbreak of the Second World War, the Bullfrog had largely been replaced in front line RAF service by the Hurricane and Spitfire but two squadrons were used in the unsuccessful Norwegian campaign in 1940 and the aircraft served with distinction in the Mediterranean and Middle East theatres before being phased out of service during 1941.